The so-called presensitized plate (abbreviated as "PS plate" hereinafter) comprises an aluminum plate coated with a light-sensitive composition in the form of a thin layer known for planographic printing plates. Generally, said aluminum plate is subjected to a surface roughening treatment, for example, by mechanical means (e.g., brush graining, ball graining), by electrochemical means (e.g., electrolytic graining), or by a combination of both means to make the surface mat. The plate is then etched with, for instance, an aqueous solution of an acid or alkali, then anodized, and, if desired, subjected to further treatment for rendering the plate surface hydrophilic. The thus-prepared support, when provided with a light-sensitive layer thereon, constitutes a PS plate. This PS plate is generally subjected to the steps of exposure for image formation, development, and gumming-up, to provide a planographic printing plate. This is mounted on a printing press, and printing is conducted therewith.
With the above sort of planographic printing plate, however, non-image areas of the planographic printing plate obtained from a positive-working PS plate through exposure and development are typically contaminated by substances contained in the light-sensitive layer which are strongly adsorbed on said areas. As a result, it becomes difficult to discriminate image areas from non-image areas in the step of retouching, or traces of retouches remain distinctly, giving an uneven printing face or, in extreme cases, leading to stains on prints which render the printing plate unusable.
To improve the above situation, several methods have been developed for preventing the above-mentioned contamination of non-image areas, in order to thereby minimize or prevent the formation of stains on prints. For example, one method comprises treating the surface of an anodized aluminum support by dipping said support in an alkali metal silicate solution, as described in U.S. Pat. No. 3,181,461. Another method comprises subbing said anodized surface with a hydrophilic cellulose containing a water-soluble zinc salt, as described in U.S. Pat. No. 3,860,426. A further method comprises subbing said surface with the sodium salt of an arylsulfonic acid, as described in British Pat. No. 2,098,627. Although the process prevents stains at non-image areas, such methods have created a new problem, in that the press life of the printing plate, i.e., the number of prints producible with said plate, is decreased by from 20 to 50% as compared to the case wherein such treatment is not applied.
In the case of a negative-working PS plate, another problem is that a planographic printing plate obtained after exposure and development has a decreased adhesion between the image-carrying areas and the support, also with the effect that the printing plate cannot be used in printing a large number of copies.
To overcome the above problems, several methods have been proposed. For instance, Japanese Patent Publication No. 6410/69 discloses a method for providing the surface of an anodized aluminum support with a thin layer of a trihydroxybenzenecarboxylic acid. According to Japanese Patent Publication No. 14337/66, said surface is provided with a thin layer of mellitic acid. Japanese Patent Publication No. 8907/63 describes a method of providing said anodized surface with a thin layer of a phosphonic acid or a derivative thereof. However, while such methods can improve the above-mentioned adhesion of image-carryign areas, they have offered a new problem. The problem is that the degree of staining of the non-image areas becomes very significant as compared with the case wherein the above-mentioned methods are not applied. In particular, such problem becomes severer with the lapse of time after preparation of the PS plate until preparation of the planographic printing plate.